Orally administrable compounds have been already known which have α4 integrin-inhibitory action and which are useful as drugs for treating or preventing an inflammatory disease in which an α4 integrin-dependent adhesion process is associated with a pathological condition. For example, Patent Literature 1 discloses a phenylalanine derivative represented by general formula (1) and a pharmaceutically acceptable salt thereof, and representative compounds thereof have the following chemical structures.

In addition, Patent Literature 1 shows results of VCAM inhibition activities (VCAM-1/α4β1 Binding Assay) and (VCAM-1/α4β7 Binding Assay).
Moreover, Patent Literature 2 discloses a phenylalanine derivative being represented by the following general formula (1) and having an R12(R13)N—X1-group at its terminal or a pharmaceutically acceptable salt thereof.

This compound was shown to have higher VCAM-1/α4β1 integrin inhibition activity in the presence of serum than the compound of Example 1 of Patent Literature 1. In addition, Patent Literature 3 also discloses a compound having α4 integrin-inhibitory action.